The positioning of a lens, mirror or similar optical element (hereafter “lens”) involves spatially locating such element within six degrees of freedom. The lens is located translationally relative to each of three orthogonal axes directions generally designated as the x(scan), y(cross-scan), and z(beam path) axes directions. The lens is also located rotationally relative to three rotational directions, generally designated as the θx, θy, and θz, directions, corresponding to angular rotation, respectively, about each of the x, y, and z axes.
Monolithic spherical lenses having one curved surface provide power magnification in two orthogonal directions, x and y, and focus parallel rays at a focal point corresponding to the center of curvature of the lens surface. Such lenses are used in laser printers, for example, for controlling beam spot size, convergence and focusing. Correct positioning of such spherical lenses in the x, y translational and θx, θy rotational directions assures alignment of the focal point and center of the lens relative to an incident beam of light coincident with the z axis. Correct location of the lens along the z axis serves to assure proper focusing of an imaged object. Considerations for locating conjugate and composite spherical lens elements are similar.
Monolithic cylindrical lenses having one curved surface provide magnification in only one direction, x or y, and focus parallel rays to a line or lens cylinder axis parallel to the other direction, y or x respectively. Cylindrical lenses are used in laser printers, for example, for beam shaping, such as for controlling x-direction or y-direction elliptical beam spot size. Cylindrical lenses may be manufactured to have a planar surface opposite the curved surface which is generally parallel to the x-y plane. Such a lens can, thus, be located in the θx and θy rotational directions by orienting the x-y planar surface normal to the incident beam z axis direction. Variations in positioning in the non-magnification direction (i.e. variations in the y direction for magnification in the x direction, and vice versa) are not critical in many applications. Thus, once correct orientation of the x-y planar surface is established, locational precision is needed only in the x or y magnification translational and θz rotational directions. Location in the z direction is left adjustable for focusing purposes.
Conventional mounts for multiple degree-of-freedom positioning of optical elements nest multiple structural components for independent relative movement, one with respect to the other, to achieve the required translational and/or rotational positioning. For example, U.S. Pat. No. 4,652,095 (Mauro) describes an arrangement of three nested stages, each having a table shiftable along rails in a respective x, y, or z translational direction by a threaded rod movable against the force of an opposing spring. The stages are nested, with the optical element mounted for movement with the table of the first stage, the first stage mounted for movement with the table of the second stage, and the second stage mounted for movement with the table of the third stage. U.S. Pat. No. 3,596,863 (Kasparek) shows an arrangement of nested flexural pivots, each providing a respective θx, θy, and θz rotational adjustment. Other examples of nested optical element mounting arrangements are given in U.S. Pat. No. 3,204,471 (Rempel); U.S. Pat. No. 4,077,722 (Bicskei); U.S. Pat. No. 4,099,852 (Kobierecki et al.); and U.S. Pat. No. 4,655,548 (Jue).
Mounting arrangements that provide multiple degree of freedom lens positioning, without nesting, are shown in U.S. Pat. No. 3,989,358 (Melmoth) and U.S. Pat. No. 4,408,830 (Wutherich). U.S. Pat. No. 3,989,358 provides independent x and y translational adjustments by micrometer spindles that are moved against knife-edges, displaced 90 degrees circumferentially about a lens retaining ring. U.S. Pat. No. 4,408,830 provides x, y, and x-y translational adjustments by moving inclined faces of screw-driven cradle elements against corresponding angled corners of a rectangular lens retainer.
As a general observation, conventional devices for achieving six-degree-of-freedom positioning of optical elements tend to be unduly complex and costly. Moreover, when used for mounting cylindrical lenses in optical systems like those of laser printers or the like, precise machining utilized to ensure correct positioning in critical directions is wasted when applied also for non-critical ones. In general, prior art mounts seek to avoid the exertion of any torque directly on the lens itself. See, for example, U.S. Pat. No. 4,909,599 (Hanke et al.)
A number of innovative solutions have been proposed for cylindrical lens mounting without undue complexity. For example, commonly-assigned U.S. Pat. No. 5,194,993 (Bedzyk) discloses an inexpensive lens mount for positioning a cylindrical lens or similar optical element in an optical system like that of a laser printer, wherein six degree-of-freedom positioning is achieved with a minimum of nesting, taking advantage of physical characteristics of the lens, and employing a push-pull mechanism for applying a biasing torque on the lens, against which adjustments in the x or y axis magnification direction and θz rotational direction are made. As another example, commonly-assigned U.S. Pat. No. 5,220,460 (Bedzyk) discloses a lens mount that applies a biasing torque against the lens in the θz rotational direction. Yet another example is given in commonly-assigned U.S. Pat. No. 5,210,648 (Bedzyk). U.S. Pat. No. 5,210,648 discloses the use of a V-shaped channel track as a base for an adjustable mount, with the V-channel providing alignment along the optical axis. A carrier contains the lens itself, providing suitable orientation in x-y directions and θx, and θy rotation, and movable along the V-channel for positioning adjustment along the z-axis. An extended bracket provides the θz rotational adjustment.
While the solutions offered in U.S. Pat. Nos. 5,194,993; 5,220,460; and 5,210,648 enable precision adjustment of lens positioning for all six degrees of freedom, the accessibility needed to make these adjustments can be a practical constraint in some situations, particularly for designs requiring compact packaging of pre-scan optical components. For example, the adjustable mount of U.S. Pat. No. 5,210,648 requires access to adjustment screws from both the front and the top of this unit. The adjustable mounts of U.S. Pat. Nos. 5,220,460 and 5,194,993 require access for adjustment from both the front and sides.
Thus it can be seen that there would be advantages to the design of an adjustable lens positioning mount having adjustments for z and θz positions accessible from a single direction.